1. Statement of the Technical Field
The inventive arrangements relate generally to hearing aids, and more particularly to methods and apparatus for achieving high fidelity hearing restoration.
2. Description of the Related Art
Human hearing generally has a frequency range of between about 20 to 20 kHz. It is well known among hearing experts that among those people experiencing hearing loss, the ability to hear will generally vary over this range of audio frequencies. Consequently, a loss of hearing has a more profound effect than merely decreasing the overall volume or perceived sound level. In fact, because hearing loss can be more significant within certain frequency ranges as compared to other frequency ranges, the diminished hearing commonly creates difficulties in allowing hearing impaired individuals to understand words which are spoken to them.
One reason for this difficulty in understanding spoken words is that loss of hearing often first begins to occur at the higher frequencies within the audible range. Since many common sounds in spoken English include these high frequency components, people with diminished hearing can often hear these spoken words only in a garbled way because key audio information is being lost in the communication channel.
The conventional solution to addressing the foregoing problem is to provide additional audio gain at those frequencies where there has been a measured loss of hearing. The most common way of providing such gain is through the use of hearing aids. Although there are several different types of hearing aids available, the basic goal of such devices is essentially the same. They amplify selected ranges of audio frequencies and provide the amplified acoustic energy to the eardrum, especially at those frequencies where there has been a measured loss of hearing. Most current digital hearing aid processors have Nyquist sampling rates that will support audio frequencies up to about 10 kHz. However, typical output transducers in such devices will generally only support audio frequencies up to about 6 kHz. Similarly, most analog hearing aids also address frequencies up to about 6 KHz, a range typically associated with “telephone” quality acoustics. However, many current approaches to high fidelity hearing restoration correctly address frequencies above 10 KHz.
The conventional method for evaluating the degree of hearing loss, and the amount of audio gain required at various frequencies, is typically determined based on threshold of hearing (TOH) testing. The TOH is lowest level sound that can be perceived by the human auditory system at a particular frequency. Thus, the testing usually involves a series of tones at various measurement points within the audible frequency range. When plotted, the data for an individual with normal hearing will generally fall on a predictable curve or contour that was first demonstrated by Fletcher and Munson at Bell Labs in the 1930s. The curves are lowest in the range from 1 to 5 kHz, with a dip between 3 and 4 kHz, indicating that individuals are generally most sensitive to frequencies in this range. The intensity of audio tones above or below this range must be raised substantially in order to create the same impression of loudness to an individual.
By comparing an individual's measured TOH to normal TOH values it is possible to make certain conclusions regarding the degree of hearing loss at each frequency. The conventional approach is to then use this information as a basis for setting the gain characteristics of a hearing aid device in an effort to increase the level of those audio frequency ranges so as to compensate for the measured degradation in hearing.
Still, many people find that hearing aids produce only marginally improved intelligibility. For example, some studies have shown that persons fitted with hearing aids only achieve 5% to 15% improvements in intelligibility. Users also tend to complain that the hearing they experience is loud and tinny. These poor results have been puzzling and continue despite many apparent improvements in hearing aid technology.
Tinnitus is another condition that affects the hearing of many individuals. Tinnitus is the perception of ringing, hissing, or other sounds in the ears or head when no external sound is present. For many, advancing age is accompanied by a certain amount of hearing impairment combined with symptoms of tinnitus. This can be explained to some extent by recent discoveries suggest that the onset of tinnitus may be linked to natural feedback mechanisms operating improperly when hearing loss occurs. Tinnitus noise is often about 50 dB over TOH and typically has frequencies above 1 kHz. Unfortunately, there is no specific treatment for tinnitus that has proven to be particularly effective.